There is so much.

* I would create an alternate trust model for TLS not using certificates.

* I would make WebSockets the default transmission medium, not HTTP (or its variants).

* I would make JavaScript strongly typed, like TypeScript, and eliminate implicit type coercion.

* I would throw away a substantial amount of HTML5. HTML should be tiny, strict, and self validating.

* There are native methods I would add to the DOM.

WebRTC, PWAs and indexeddb